The utility of many semiconductor optoelectronic devices is based on their efficiency in converting input electrical power into output optical power, as for example, in light-emitting diodes (LEDs) or laser diodes; or conversely, on their efficiency in converting input optical power into output electrical power, as for example, in photovoltaic solar cells or photodiodes. This invention relates both to 1. semiconductor device designs in which these conversions can be effected more efficiently, and 2. processes for fabricating such devices.
Light-Emitting Diodes
Light-emitting diodes (LEDs) are semiconductor devices that convert electrical power into optical power. They are used for, among other things, displays, indicator lights, and fiber optic light sources. The operation of a light emitting diode is based on physical phenomena that are well understood and quantitatively characterized.
In LEDs, light of a specified wavelength range is generated by radiative processes in which an excess (non-equilibrium) number of minority charge carriers (i.e., electrons in material of p-type conductivity; or holes in material of n-type conductivity) combine with majority charge carriers (i.e., holes in material of p-type conductivity; or electrons in material of n-type conductivity), emitting photons with a characteristic energy close to that of the bandgap of the material. This radiative phenomena is especially efficient in certain semiconductors, and in particular in III-V compound semiconductors such as gallium arsenide (GaAs), indium phosphide (InP), gallium nitride (GaN), their related alloys such as aluminum gallium arsenide (AlGaAs), aluminum gallium indium phosphide (AlGaInP) and indium gallium nitride (InGaN), in II-VI compound semiconductors such as zinc selenide (ZnSe), zinc sulfide (ZnS) and related materials, and less efficiently in other semiconductors such as silicon carbide (SiC). The emission spectrum or color of the generated light is determined to a large degree by the bandgap of the semiconductors and to a lesser degree by impurity doping.
One of the most efficient means to produce an excess concentration of minority carriers in a semiconductor utilizes the mechanism of minority carrier injection. By applying a voltage to a junction formed between two judiciously chosen materials, at least one of which is a semiconductor, an excess concentration of minority carriers in an excited energy state can be created in the semiconductor. These injected excess minority carriers combine with majority carriers (which are present due to impurity doping of the semiconductor). A fraction of the recombination phenomena is radiative in that a photon of light is generated for some fraction of recombination events.
The most common device for the purpose of injecting minority carriers is the p-n junction diode which designates the structure formed between a material which exhibits p-type conductivity (conduction primarily by positive charge carriers, i.e., holes) and a material which exhibits n-type conductivity (conduction primarily by negative charge carriers, i.e., electrons). Other structures for injecting minority carriers are metal-semiconductor junctions (Schottky barriers), and metal-insulator-semiconductor (MIS) junctions. Although Schottky or MIS junctions are not as efficient for minority carrier injection as p-n junctions, they are sometimes used for LEDs in cases where the choice of semiconductor material makes it difficult or impossible to form a p-n junction Some of the minority carriers so injected into a luminescent semiconductor material such as GaAs combine with majority carriers (which are present at a relatively high concentration due to impurity doping) through various radiative (i.e., luminescent or light-emitting) processes. Other minority carriers combine with majority carriers by non-radiative processes and such processes represent a loss in LED efficiency.
The light generated by the radiative processes is isotropic in the sense that there is no preferred direction of the luminescence. Because of the high refractive index of most semiconductors, only a relatively small fraction (5 to 10%) of the generated light can escape through the front surface. Most of the light which is not at near-normal incidence to the front surface is internally reflected from the front surface. It is only the fraction of generated light that is transmitted through the front surface that constitutes the useful optical output of the LED. The remaining light is confined and undergoes multiple internal reflections and is mostly re-absorbed in the epitaxial layers, in the substrate, or at metal contacts. This re-absorption of luminescence is a major loss factor in LEDs.
To assess the importance of various loss mechanisms, it is instructive to view the total or overall efficiency .sub.TOT of an LED as the product of three efficiency components: EQU .sub.TOT =.sub.INJ RAD C
The injection efficiency is the fraction of electrical current which constitutes the injected minority carriers. In hetero-junction LEDs, the injection efficiency can be close to 1. The radiative efficiency is the fraction of injected minority carriers which undergo radiative combination with emission of a photon. In well-developed LED materials, such as GaAs, the radiative efficiency can also be close to 1. In conventional LEDs, the coupling efficiency is typically on the order of 0.01 to 0.1. As described previously, this is a consequence of the isotropic nature of the luminescence and the high refractive index of semiconductors, such that only a small fraction of the luminescence escapes through the front surface of the LED. It is obvious that any further significant improvements in LED efficiency must come by way of improvements in the coupling efficiency.
Approaches to reduce the coupling losses include structuring, "lensing" or texturing the front emitting surface of the LED. In practice, these techniques are too complicated for low-cost LED production or do not sufficiently improve over-all LED efficiency to justify their use in commercial LEDs.
In its application to LEDs, the invention relates primarily to improvements in coupling efficiency. It is an object of the invention to incorporate features in the LED structure, which result in dramatic enhancement of the coupling efficiency. In particular, a reflecting layer sandwiched between the luminescent semiconductor layers and the supporting semiconductor substrate and which we term a "buried mirror" is used to improve the LED coupling efficiency. The buried mirror reduces absorption losses and enhances so-called photon recycling effects. Photon recycling refers to phenomena whereby luminescent photons are absorbed in the semiconductor material, thus generating new minority carriers which in turn recombine radiatively to generate photons. These additional photons produced by photon recycling effects can make a significant contribution to the optical power output of the LED. The buried mirror design effectively enhances and exploits recycling phenomena in LEDs as described by SCHNITZER et al. in Applied Physics Letters 62, 2 1993.
In the buried mirror LED design, the thickness of the epitaxial device layers can be adjusted to form a resonant optical cavity. Such a resonant cavity can be used to provide narrower emission angles, narrower emission spectral widths, increase quantum efficiencies, and increase modulation speeds.
Solar Cells
A solar cell is a device for converting light into electrical power. The invention can be applied to solar cells which are comprised of epitaxial layers as is common with GaAs-based solar cells, or less commonly, with silicon solar cells. The use of a buried mirror can improve the performance of a solar cell in several ways. The absorption of light in the solar cells can be enhanced since weakly absorbed light that is normally transmitted into or through the substrate is reflected back into the active layers of the solar cell. Thus, a greater fraction of incident light is absorbed near the semiconductor junction. Photogenerated minority carriers generated close to the junction have a high probability of being collected, thereby reducing minority carrier recombination losses. The enhanced absorption provided by the buried mirror also permits the use of thinner absorbing layers since the optical absorption path is at least doubled. Furthermore, if the top surface of the solar cell is textured, the buried mirror can provide significant light trapping wherein the optical absorption path is increased by a factor much greater than 2. Photon recycling effects can then be very efficiently utilized in improving the device performance. The thinner absorbing layers can also sustain higher impurity dopings which will increase the open-circuit voltage of the solar cell. Solar cells based on thin device layers also have an inherently higher collection efficiency since the minority carriers are photogenerated close to the collecting junction. The solar cell performance is less sensitive to material quality which reduces fabrication costs and increases radiation resistance. Radiation hardness is important for space power applications.
Photodiodes
The advantages of a "buried" metallic reflector for photodiodes are similar to the advantages for solar cells. One additional advantage is related to the bandwidth or response time of a photodiode. The response time, or equivalently the frequency bandwidth, of a photodiode is in part limited by the transit time photogenerated carriers drift or diffuse to the collecting junction. By using a buried mirror, the optically absorbing layers of the photodiode can be reduced without the concomitant loss in photocurrent. A thinner absorbing layer reduces the transit time of photogenerated carriers thereby reducing the response time of the photodiode. The thickness of the optically absorbing semiconductor layer can be adjusted to form a resonant optical cavity. By forming a resonant cavity, the quantum efficiency of a photodetector can be increased by a factor of almost two. The spectral response can also be made more selective by use of such resonant cavities.
Laser Diodes
Vertical cavity surface emitting lasers are comprised of an optical cavity formed between one or two Bragg reflectors. The Bragg reflector is a multilayer stack of alternating dielectric or semiconductor materials of distinct refractive indexes and quarter wavelength optical thicknesses. For semiconductor devices, a Bragg reflector is made by growing multiple epitaxial layers of lattice-matched semiconductor materials. There are three criteria for selecting materials as components of a Bragg reflector. The materials must be relatively non-absorbing to light at the wavelengths of interest. The materials must exhibit a significant difference in refractive index in order to effect high reflectivity with a reasonable number of layers. The materials must be chemically compatible and have a close lattice mismatch in order that the epitaxial semiconductors of device quality may be formed over the Bragg reflector.
The buried reflector approach described herein provides an alternative structure when the above criteria cannot be easily met in epitaxial devices. For example, for some material systems of much interest, such as quaternary InP-based semiconductors, the refractive index change between the component layers of the Bragg reflector is not sufficient to effect an efficient reflector. In optoelectronic devices that operate at long wavelengths, the requirement of quarter-wavelength thick layers leads to impractically thick Bragg reflectors.
Permeable Base Transistors
The permeable base bipolar transistor is comprised of a metal or metallic compound base layer sandwiched between two semiconductor layers that serve as the emitter and collector of the transistor. These transistors are expected to perform at exceptionally high switching speeds compared to conventional bipolar transistors which utilize a semiconductor base region. The proposed fabrication technology, wherein a single-crystal semiconductor layer is formed over a metal film, can produce the device structures required for such permeable base bipolar transistors.
Epitaxial Lateral Overgrowth on Specially Masked Substrates
A further object of the invention relates to crystallization technique for forming single-crystal semiconductor layer(s) over metal layers or combinations of metal and dielectric layers. It is generally not possible to crystallize a device-quality, single-crystal semiconductor layer directly on a metal or dielectric film. This is due primarily to the fact that the metal or dielectric film are neither monocrystalline nor lattice matched to any useful semiconductor materials. Other methods for forming semiconductor layers on oxides often involve some type of selective melting of a deposited amorphous or polycrystalline overcoating of the oxide and subsequent controlled recystallization. While these are workable for silicon and germanium, they are not feasible with many compound semiconductors due to the incongruent vaporization of compound semiconductors. An important facet of this invention is the development of a relatively low-temperature liquid-phase lateral epitaxy process for forming single-crystal epitaxial layers on reflective masked substrates wherein the epitaxial semiconductor crystals so formed have dimensions that are useful and optimal for semiconductor devices.